The broad purpose of the proposed research is to further the understanding of the mechanical behavior of compact bone tissue at the micromechanics level. Micromechanics modeling techniques are used extensively and quite successfully in elucidating fundamental structure/property relationships and failure mechanisms for fiber reinforced composite materials. Their potential remains largely untapped, however, in studying bone and other biological materials. A micromechanics model consists of a small repeatable unit cell of the material that explicitly contains each discrete material constituent. This unit cell is intended to capture the essential microstructural features of the material in an average sense. The microstructure of Haversian compact bone tissue resembles that of a fiber reinforced composite, so this particular bone tissue type offers a natural starting point for applying composite material micromechanics methods. For Haversian bone, secondary osteons form the basic structural element and represent the fiber component. Micromechanics modeling will make possible characterization and prediction of the effects of histomorpho- metric parameters such as porosity, percent Haversian area, osteon type (collage fiber orientation), mineralization, and density on the macroscopic mechanical properties of bone tissue. Thus, knowing the effects of various pathological conditions on these parameters, the model will allow an assessment of the corresponding effects on mechanical behavior. The specific goals of the proposed research are to develop and test a micromechanics model of Haversian compact bone. Various methods for explicitly incorporating porosity into the model will be examined. The Collaborator on the project will provide detailed experimental results reporting elastic modulus measurements as a function of several histomorphometric parameters (including porosity). The model will be adapted, improved, and evaluated through direct comparison with this data. Most of the effort in this one-year project period will be needed for model development and assessment. Successful completion of the work will lay the foundation for follow-up studies involving further model development as well as extensive experimental testing for more definitive model verification. The current project is necessary to establish the feasibility of the method to warrant such longer-term study. The ultimate benefit of this line of research is a more complete and deterministic understanding of the basic science of bone mechanics. The emphasis in this initial effort is on porosity because of its critical role in the many major health problems arising from osteoporosis, whether through disuse, aging, or metabolic disorders.